dillon1000/react

[Scheduler] Test browser implementation details (#16198)

The Scheduler implementation uses browser APIs like `MessageChannel`,
`requestAnimationFrame`, and `setTimeout` to schedule work on the main
thread. Most of our tests treat these as implementation details;
however, the sequence and timing of these APIs are not precisely
specified, and can vary wildly across browsers.

To prevent regressions, we need the ability to simulate specific edge
cases that we may encounter in various browsers.

This adds a new test suite that mocks all browser methods used in our
implementation. It assumes as little as possible about the order and
timing of events. The only thing it assumes is that
requestAnimationFrame is passed a frame time that is equal to or less
than the time returned by performance.now. Everything else can be
controlled at will.

It also includes Scheduler-specific invariants, e.g. only one rAF
callback can be scheduled at a time.

It overlaps slightly with the existing SchedulerDOM-test suite, which
also mocks the browser APIs, but exposes a higher-level set of testing
primitives. I will consolidate the two suites in a follow-up.